In many instances, it is desirable to identify the surface properties of a workpiece including the type of material from which a workpiece is constructed. Conventionally, a technician could determine the type of material from which a workpiece was constructed and, consequently, the surface properties of the workpiece, by visual inspection of the workpiece. However, workpieces are commonly painted and textured such that workpieces constructed of different types of materials appear visually identical, but have dramatically different surface properties.
For example, some modern military aircraft employ low-observable technology to decrease, or eliminate, the radar signature of the aircraft, thereby significantly increasing the probability of survival of the aircraft on a mission. For example, the surface or skin of the aircraft is typically comprised of a plurality of parts, at least some of which are conductive.
In order to reduce the radar signature of the aircraft, conductive parts are configured so that they do not reflect radar energy to the radar source. In particular, strategically placed radar absorbing coatings cover those parts that would otherwise reflect radar energy to the radar source. During the service life of the aircraft, however, a number of the parts must be periodically replaced. In order to maintain the reduced radar signature of the aircraft, all parts must be replaced with parts which have the same surface properties as the corresponding parts that were removed.
Aircraft, which employ low observable technology to reduce its radar signature during stealth missions, can also be configured for conventional missions in which one or more parts which are coated with Radar Absorbing Material (RAM) during stealth missions are constructed, instead, from other materials, such as metallic or fiberglass materials. These parts, such as door panels, typically appear visually identical to the corresponding parts coated with RAM since, in many instances, the conventional parts are painted the same as the corresponding parts coated with RAM. However, these conventional parts have significantly different electromagnetic properties and, consequently, significantly different effects on the radar signature of the aircraft. Thus, if conventional parts comprised of metallic or fiberglass materials are inadvertently installed on an aircraft intended for a stealth mission, the radar signature of the aircraft will be increased and the probability of survival of the aircraft decreased.
In order to ensure that the parts installed on an aircraft have the proper surface properties for their intended missions, i.e., parts coated with RAM are installed on aircraft configured for stealth missions and parts comprised of more conventional materials, such as metallic or fiberglass materials, are installed on aircraft configured for conventional missions, maintenance crew members can identify the serial number of each part and, based on a tabular listing of the characteristic properties of each part by serial number, identify the type of material forming the part. However, the time and expense required to maintain and update a tabular listing of the characteristics of each part are substantial. In addition, the time required for maintenance crew members to cross-reference each part in the tabular listing is significant and, in an effort to more rapidly launch an aircraft, could be ignored.
Another method to identify that the proper type of parts have been installed on an aircraft is to fly the aircraft, following its initial configuration, in a friendly environment and to observe the radar signature of the plane. By monitoring the radar signature of the aircraft, it can be determined if the proper parts were installed on the aircraft. However, the resulting radar signature of an aircraft would not specifically identify the individual part or parts which were constructed from the improper material. Consequently, maintenance crew members would be required to cross-reference the serial number of each part in order to ensure that the parts constructed of the improper material were replaced. In addition, this method of detecting an improperly configured aircraft is also relatively expensive due to the cost of the diagnostic flight and the time required to observe and analyze the radar signature of the aircraft.
A surface reflectivity sensor is marketed by Millimeter Wave Technology, Inc. which can distinguish between a metal surface and a RAM coated surface. In particular, the surface reflectivity sensor include a pair of antennas which transmit microwave signals to a workpiece and which measure the microwave signals reflected from the workpiece. Based upon the intensity of the reflected microwave signals, the surface reflectivity sensor determines if the workpiece is reflective or absorptive of microwave energy. Based upon the absorption of microwave energy by the workpiece, the surface reflecting sensor can determine if the part is coated with RAM. However, the surface reflectivity sensor cannot distinguish between a RAM coated material and a dielectric material since both materials appear equally reflective to the surface reflectivity sensor. In addition, the surface reflectivity sensor is relatively large and cumbersome since it typically includes a 20-30 pound external power supply. The surface reflectivity sensor also generally requires a 4-inch diameter measurement area so as to inhibit testing of parts having limited lateral access.